The present invention relates to electron discharge devices of the camera tube type and more particularly to a zoom focus and deflection assembly for such electron discharge devices.
Focus and deflection assemblies are electron optical devices used to provide the magnetic fields needed to operate image dissector tubes and other electron discharge devices of the camera tube type including vidicons, orthicons, other types of television pickup camera tubes, variable magnification image intensifier and image converter tubes.
In general a focus and deflection assembly includes three wire wound coils or, in some cases, printed circuit conductors, arranged so that the axis of each coil is orthogonal to the other two coil axes. The three coils may be, or may not be contained within a magnetic shield.
The usual construction practice has two saddle wound deflection yokes contained within a solenoid focus coil. There are cases where the innermost coil is the focus solenoid and the deflection yokes are on the outside.
The focus coil provides a magnetic field oriented essentially parallel to the tube axis of rotational symmetry. Focus coils typically have a 2:1 to 5:1 length to diameter ratios and provide fields of 2 to 5 mT (millitesla) when used with vidicons or image dissectors. This field, in conjunction with the electrostatic potentials in the tube focuses the flow of electrons from the gun to target in a vidicon or from the photocathode to the aperture in the image dissector.
The deflection yokes act orthogonal to the focus field and are typical 1/10 or less the strength of the focus field. The deflection fields cause the electron beam of a vidicon to scan the target and cause the image to move with respect to the sampling aperture in an image dissector.
In present image dissector tubes and deflection assemblies, the sampling aperture is selected at the time of tube construction. The sampling aperture size can be changed only by substituting another tube or by using expensive tubes having multiple apertures. In neither case can the size of the aperture be smoothly and continuously adjusted. Multi-aperture tubes are subject to cross talk between apertures.
Deflection assemblies used with image dissectors exhibit non-linear deflection in terms of current versus position in the image.